Hydrocarbon alkylation is widely used in the petroleum refining and petrochemical industries to produce a variety of useful acyclic and cyclic hydrocarbon products which are consumed in motor fuel, plastics, detergent precursors, and petrochemical feedstocks. Much of the installed base of alkylation capacity uses liquid phase hydrofluoric acid, generally referred to as HF, as the catalyst. The use of HF in these applications has a long record of highly dependable and safe operation. However, the potential damage from an unintentional release of any sizable quantity of HF and the need to safely dispose of some byproducts produced in the process has led to an increasing demand for alkylation process technology which does not employ liquid phase HF as the catalyst.
Numerous solid alkylation catalysts have been described in the open literature. However, these catalysts appear to suffer from unacceptably high deactivation rates when employed at commercially feasible conditions. While some catalysts have a sufficiently useful lifetime to allow the performance of alkylation, the rapid change in activity results in a change in product composition and also requires the periodic regeneration of the catalyst with the accompanying removal of the reaction zone from operation. It is very desirable to provide a continuous process for alkylation which is not subjected to periodic reaction zone stoppages or variation in the product stream composition.
In hydrocarbon processing, continuous catalytic processes commonly use transport reactors. In a transport reactor, the catalyst bed as a whole moves and is transported with a fluid phase. Thus, a transport reactor can be contrasted with a fixed bed catalytic reactor and with an ebulliated bed catalytic reactor. In a fixed bed reactor the catalyst particles do not move, and in an ebulliated bed reactor the catalyst particles are suspended in a fluid but the settling velocity of the catalyst particles balances the fluid upflow velocity so that the catalyst bed as a whole is not transported with the fluid phase. Although it is generally the case that the direction of catalyst flow through a transport reactor is upward, the direction may also be downward, horizontal, a direction that is intermediate between vertical and horizontal, or a combination of these directions.
When the direction of catalyst flow through a transport reactor is upward, the transport reactor is often called a riser-reactor. Riser-reactors are commonly used in hydrocarbon processing, such as fluidized catalytic cracking and more recently in motor fuel alkylation. In a common arrangement, a fluid hydrocarbon reactant engages a solid hydrocarbon conversion catalyst at the bottom of a riser-reactor and transports the catalyst in a fluidized state up the riser-reactor. During the ascent through the riserreactor, the catalyst promotes certain desired conversion reactions among the reactants in order to produce desired products. A stream of catalyst and hydrocarbon products, byproducts, and unreacted reactants if any discharges from the top of the riser-reactor into a separation zone. The hydrocarbons and the catalyst disengage in the separation zone, with the hydrocarbons being withdrawn overhead for recovery and the catalyst dropping by gravity to the bottom of the separation zone. Despite some deactivation that may have occurred to the catalyst in the riser-reactor, some of the catalyst that collects at the bottom of the separation zone may have enough residual activity that it can be reused in the riser-reactor without first being withdrawn from the separation zone for regeneration. Such still active catalyst is recirculated through a recirculation conduit from the bottom of the separation zone to the bottom of the riser-reactor, where the catalyst contacts reactants again.
Most commercial alkylation catalysts, however, require periodic regeneration of the catalyst with the accompanying removal of the catalyst from the reaction zone for regeneration. Depending on the particular catalyst and on the nature and degree of the deactivation, the periodic catalyst regeneration may be in the liquid phase or in the vapor phase. Liquid phase or "mild" regeneration comprises contacting the catalyst with a liquid phase hydrocarbon, which is commonly a feed hydrocarbon, such as isobutane, with dissolved hydrogen. Vapor phase or "severe" regeneration, on the other hand, comprises contacting the catalyst with hydrogen gas at a higher temperature and/or for a longer time than liquid phase regeneration. Vapor phase regeneration, which is also called "hydrogen stripping," is a more intense regeneration than liquid phase regeneration. Whereas liquid phase regeneration helps to remove alkylate, light byproducts, and lightly sorbed contaminants from the catalyst, vapor phase regeneration helps to remove more strongly sorbed species, such as heavies which, if allowed to accumulate on the catalyst, would deactivate the catalyst and would cause the alkylate yield to decline. As used herein, the collective term "heavies" refers to oligomers having twelve or more carbon atoms. Because liquid phase regeneration may not remove these strongly sorbed oligomers, vapor phase regeneration is usually performed after a specified number, such as three or four, of liquid phase regenerations. But, in processes where the rate of catalyst deactivation or rate of yield loss is extremely rapid, vapor phase regenerations may be done either after each liquid phase regeneration or even instead of liquid phase regeneration. Thus, vapor phase regeneration can be a critical step in a continuous alkylation process, regardless of whether the alkylation process includes liquid phase regeneration.
In contrast to regeneration in the vapor phase, the alkylation reactor generally operates in at least partially liquid phase conditions, including supercritical conditions. Consequently, catalyst particles that are withdrawn from the alkylation reactor for regeneration contain entrained liquid hydrocarbons, both in the pores of the catalyst particles and in the interstitial volume between the catalyst particles. Although in theory this entrained hydrocarbon liquid could remain with the catalyst particles and be removed along with the strongly sorbed species during vapor phase regeneration, it is preferred as a practical matter to remove these liquid hydrocarbons prior to vapor phase regeneration in order to simplify the handling of the vapors that are employed in vapor phase regeneration and to exclude the need for separating liquids from the vapor phase regeneration gases. Thus, prior to vapor phase regeneration, the entrained liquid is removed from the catalyst in a process that is known as dewetting.
Subsequent to vapor phase regeneration, the catalyst particles must, of course, ultimately be returned to the alkylation reactor. But adding the vapor phase regenerated catalyst directly to the alkylation reactor, however, creates several problems. These same problems also arise when adding dry fresh catalyst as makeup directly to the alkylation reactor. In either case, directly contacting the catalyst with liquid hydrocarbons generates the heat of adsorption of the liquid hydrocarbons on the catalyst particles. If not removed from the alkylation reactor, this heat can cause side reactions that produce undesirable byproducts. Moreover, the released heat can vaporize hydrocarbons that are preferably maintained as liquids, not vapors, at alkylation conditions. Thus, the released heat necessitates the use of extra equipment to cool, condense, and recycle the vaporized hydrocarbons. In addition, the vaporization of hydrocarbons can cause pressure imbalances in vessels through which catalyst particles flow that can stop or reverse the direction of catalyst flow between the vessels.
Thus, a method is sought for wetting vapor phase regenerated or fresh catalyst particles with liquid hydrocarbons in a manner that does not cause undesirable side reactions, that does not require the use of additional condensers and other equipment for recycling hydrocarbon vapors, and that does not impede catalyst flow.